(1) Field of the Invention
The present invention relates to a process for the preparation of a copolymer comprising D-3hydroxybutyrate (hereinafter referred to as "component B") and D-3-hydroxyvalerate (hereinafter referred to as "component V").
(2) Description of the Related Art
Poly-3-hydroxybutyrate (hereinafter referred to as "PHB") is a thermoplastic polymer, which is formed and accumulated as an energy-storing substance in cells of various microorganisms and shows an excellent biological decomposing property and an excellent living body compatibility, and therefore, the polymer attracts an attention as environment-preserving "clean" plastics and applications to a medical material such as a surgical thread or a broken bone-fixing material, or a gradually releasing system for gradually releasing a medicine or an agricultural chemical have been long investigated and expected. Recently, synthetic plastics have raised serious problems concerning environmental pollution and exhaustion of natural resources, and therefore, PHB has attracted attention as biopolymer not depending on petroleum. However, the melting point of PHB is close to the heat decomposition and PHB is brittle, and because of these defects, PHB can hardly be practically used.
Investigations have been made in the art for solving these problems, and polymers comprising component B and component V, which have a reduced melting point, an improved softness and an improved processability, and processes for the preparation of these polymers have been proposed (see Japanese Kokai Patent Publication No. 57-15093, Japanese Kokai Patent Publication No. 59-220192, Japanese Kokai Patent Publication No. 63-269989 and Japanese Kokai Patent Publication No. 01-69622). In these patent publications, there are processes in which copolymers are prepared by contacting cells of Alcaligenes eutrophus with a mixture of glucose-propionic acid, glucose-isobutyric acid and glucose-valeric acid under propagation-restricting conditions as attained by limiting nitrogen or phosphorus. These processes, however, are industrially disadvantageous because the manufacturing cost is high.
Accordingly, the present inventors made investigations with a view to developing a process capable of producing a copolymer comprising component B and component V more stably at a low manufacturing cost, and as the result, the present inventors have now completed the present invention.